The present invention is related to linear regulator circuits, and, more particularly, to a linear regulator circuit having a stable compensation circuit and method that is particularly useful when used in automotive applications.
A traditional regulator integrated circuit 100 for use in automotive applications is shown in FIG. 1. In a typical example, transistors M1 and M2 are 5V NMOS devices, transistors M4 and M5 are HV-NDMOS devices, and transistors M3 and M6 are HV-PMOS devices. Transistors Q1 and Q2 are bipolar transistors. An external 14.4V battery voltage is applied at node 102, an internal 5V battery voltage is applied at node 104, and a bandgap voltage VBG is applied at node 106. An operational amplifier or gm stage 108 is used in the feedback loop. The output of the regulator 100 drives the output load as shown.
In the regulator loop of the circuit shown in FIG. 1, there are three poles and two zeros that are the main contribution to stability as listed below:                P0=Req3*Co; P1=Req1*Cc; P2=Req2*Ceq;        Z0=ESR*Co; Z1=Rc*Cc.        
where:
Co: Output capacitor,
Cc: Internal compensation capacitor,
Ceq: equivalent capacitor in the gate node of M6,
ESR: equivalent series resistor of Co,
Req1: output resistor of gm stage,
Req2: equivalent resistor in the gate node of M6, and
Req3: equivalent resistor in the output node of the regulator.
The problem with the low drop-out regulator 100 shown in FIG. 1 is that a different load current results in different Idr1 and Idr2 currents. Equivalent resistors Req2 and Req3 are also different for different load currents, and hence poles P0 and P2 are undesirably variable.
In a practical design, zero Z1 is constant and used to cancel pole P2. Poles P0 and P1 are dominant poles, but P1 is constant while P0 is variable. Therefore, if Z1=P2 are under light load conditions, then regulator 100 tends to over-compensate under heavy load conditions. This is because poles P2 and P0 are much farther out in frequency in heavy load conditions than in light load conditions, while Z1 is quite low in frequency. If Z1=P2 under heavy load conditions, then regulator 100 tends to under-compensate under light load conditions, because poles P2 and P0 are much lower in frequency in light load than in heavy load while Z1 is quite high. And so a stable regulator requires that the capacitance and ESR of the output capacitor should be in a very limited range to avoid worsening over-compensation or under-compensation any further.
What is desired, therefore, is a low drop-out regulator that can be easily compensated without any of the drawbacks such as load current sensitivity and the requirement of a limited output capacitance range that is present in prior art regulators.